Marina De Franceschini
The Pantheon in Rome:
New Images of Light phenomena.
The Arch of Light.
Astronomic Observatory of Genoa, Italy
april 12-13th, 2014
16° Seminar of Archaeoastronomy
ALSSA: Ligurian Association of Archaeoastronomical Studies
Fig. 1 - The interior of the Pantheon with the magniﬁcent dome, 43.4 meters in diameter, with ﬁve rows of 28
lacunars each. On top is the oculus, its only source of illumination. (Photo by Francesco Lerteri).
!The Pantheon of Rome - originally built by Agrippa and rebuilt by Domitian - after
another ﬁre was rebuilt in its present form by Hadrian, in 125 AD; it is one of the few
Roman monuments studied from the point of view of Archaeoastronomy (ﬁg. 1). It has the
largest unreinforced concrete dome of the world (a record shared with the dome of
Brunelleschi in Florence); its diameter of 43.4 meters is equal to its height, as if the
building was built around an imaginary sphere (ﬁg. 2).
Fig. 2 - The imaginary sphere inside
(graphic processing from Ortophoto)
Fig. 3 - The window grate above the large
bronze door, both original (Photo MDF)
Fig. 4 - The Vault above the entrance corridor
ends with an arch that interrupts the molded
cornice at the base of the dome (photo MDF)
Imaginary sphere inside the Pantheon
height of the Dome = diameter 43,4 meters
!On top of the dome there is a large circular opening, the oculus, whose diameter is 9
meters; it works as a keystone on which converge the thrusts of the dome itself: it would
collapse without it. The dome is one of the masterpieces of ancient Roman architecture,
built with decreasing thickness and lighter materials (including pumice) as you progress
upwards. The oculus is the only source of illumination of the building, and is surrounded
by ﬁve rows of twenty eight lacunars each, decreasing in size.
!The entrance of the Pantheon still has the large imposing original bronze door,
weighing several tons, above which there is a window with a grate (ﬁg. 3); then follows a
short corridor covered by a barrel vault, ending inside with an arch that interrupts the
molded cornice of the dome (Figs. 4 and 15).
!The entrance is oriented towards north, with a small difference of a few degrees;
this means that on true local noon the rays of the Sun pass through the door and
illuminate the porch at the entrance, but this occurs only at certain times of the year.
!Since during the year the Sun has different heights, the dome inside the Pantheon
functions as a true spherical Sundial: the oculus creates a disk of light (Fig. 5) which moves
gradually from left (east) to right and illuminates the entrance at true local noon, and then
continues towards right (west). Depending on the season, the disc of light has a different
Fig. 5 - The Sun has different heights during the year, therefore on true local noon also the
disk of light inside the dome has a different heights (graphics processing from Ortophoto)
Height and width of
the circle of light
change according to
height above the entrance, indicating the passage of time; this was also a way of checking
the accuracy of the Calendar.
!The dome was a symbolic representation of the Heavenly Vault, and its molded
cornice represents the Equator; during Summer (between the two Equinoxes) the disc of
light disc hits the walls below the cornice and reaches the ﬂoor on Summer solstice, while
in Winter it always remains above the cornice, illuminating only the lacunars of the dome
(again Fig. 5 and Fig. 6).
!The Pantheon has the following coordinates: 41° 53' 55"North and 12° 28' 37" East.
At true local noon we have the following phenomena:
- On Winter Solstice, December
23, the disc of light appears high
up inside the dome, with an
angle of 24° (ﬁg. 6a and 7).
- On the occasion of the two
Equinoxes, March 21st and
September 23rd, the Sun has an
angle of 48°: the disc illuminates
the grate of the window above
the entrance door, and the rays
hit the pavement of the front
porch (ﬁg. 6b and 8).
Spring and Autumn
mar. 21st, sept. 23rd
Fig. 6a-b-c-d - The different heights and angles of the Sun
during major astronomical events of the year (graphics
processing from Ortophoto)
- On April 21, Dies Natalis Urbis
Romae, the Sun illuminates the
entrance door, with an angle of
60° (ﬁg. 6c and 9).
- Finally, on June 21, Summer
Solstice, the sun casts a large
disc of light on the ﬂoor, of the
diameter of 9 meters, with an
angle of 72° (ﬁg. 6d and 10).
Dies Natalis Urbis
Fig. 7 - Winter Solstice: the disk of light illuminates the upper part of the dome (photo MDF).
Fig. 8 - Equinox: the disk of
light illuminates the cornice
indicating the Equator,
passing through the grate
above the entrance door
Fig. 9 - April 21st, Dies Natalis of Rome:
the disk of light illuminates the entrance
door and the front porch (photo MDF).
!In 2012, a ﬁlm made by Nick Glass for CNN1 showed the movement of the disk of
light inside the dome of the Pantheon, which at one point perfectly coincides with the arch
of the barrel vault above the entrance
corridor, creating a prodigious Arch of
Light. According to what was said in
the movie, the phenomenon occurred
on the Equinox, but an inspection
carried out in those days showed me
that it was not true; the Arch of Light
did not appear also on April 21 –
another key date of the building.
!To solve the problem I asked (as
always) the help of archaeoastronomer
Giuseppe Veneziano, asking him to
calculate in which days the sunlight hit
the arch above the door. His response
was that the phenomenon occurred
twice during the year: during Spring in
the days between April 7th and 10th,
and in late Summer, between September
2nd and 5th.
!On April 7th 2014, I went to the
Pantheon, and I saw that the
calculations of Giuseppe Veneziano
were accurate and correct: at 1:02 pm
the Sun casts an Arch of Light perfectly
matching the arch of the barrel vault
above the entrance corridor (Fig. 11).
backstory.revealer.pantheon.cnn.html, minute 4:18
Fig. 10 - Summer Solstice: the Sun creates
a large circle of light on the ﬂoor, with a
diameter of nine meters, as the oculus
Fig. 11 - April 7th, 2014: the Sun creates an Arch of
Light perfectly matching the one above the entrance
corridor (photo MDF).
!On September 2nd and 3rd 2014, I went again to the Pantheon to verify the
calculations, but in those ﬁrst two days of the period indicated by Giuseppe Veneziano the
Arch of Light did not coincide perfectly with the barrel vault. On September 4, ﬁnally, I
could again observe the Arc of light perfectly matching, at 1:00 pm (ﬁg. 12).
Fig. 12 - September 4th, 2014: the phenomenon of the Arch of Light is
repeated for the second time during the year (photo by Francesco Lerteri).
! ! On April 7th, I also discovered another 'special effect' of light, which regularly
occurred also in September: the Sunlight passes through the main door and illuminates the
ﬂoor of the porch outside, drawing a square of light which perfectly coincides with a large
square of marble of the ﬂoor, in the center of which is a circle of granite (ﬁg. 13). As often
happens, this is a light phenomenon which was possible to discover out only by direct
observation on the spot; we never thought of calculating a luminous effect of this kind. In
practice, the large squares of the ﬂoor, inside the Pantheon and outside in its porch,
function as a ﬂat Sundial: each of them is hit by light on a different time of the year, and
works as a Calendar.
!Summarizing, the bright disk of light inside the Pantheon had a different height
depending on the Season and illuminated the main door on important occasions: during
astronomical events like the days of the Equinox, or symbolic dates as April 21, Dies
Natalis of Rome. During the summer months, the Sun creates a big bright disk of light on
the ﬂoor, reaching the extreme point inside the building during the days of Summer
!The Arch of Light above the door (which was not noticed by any other study of
Archeoastronomy) is the most extraordinary and unique luminous phenomenon among
those occurring in the Pantheon, because it perfectly matches the arch of the barrel vault.
This perfect arch certainly can not be coincidental: it is the result of extremely accurate
astronomical calculations. As further evidence of the accuracy with which the building
was planned and constructed, it is important to point out that the diameter of the oculus -
Square of Lightmartching the square of
marble in front of the main door
Fig. 13 - April 7th and September 4th, 2014: the Sun passing through the entrance door
draws a square of light that coincides with a square of marble of the ﬂoor (photo MDF)
nine meters - is exactly equal to the width of the arch above the barrel vault of the entrance
!The Arch of Light had the purpose of signaling two moments particularly
important during the year: which were the religious celebrations of the Roman calendar
occurring in those days of April and September?
!During Spring - between April 7 and 10 - there were celebrations in honor of
goddess Cybele. Her cult was imported to Rome transporting there a black stone from
Pessinus (in Asia Minor), after the Sibylline books had prophesied that the Romans would
defeat the Carthaginians by ingratiating their goddess, who was Cybele.
!In Roman cult, Cybele was identiﬁed with the Magna Mater, and in her honor in 191
BC on the Palatine was built a temple, which burned down several times. In 3 A.D. the
temple was rebuilt for the third time by Augustus: it was near the House of Augustus on
the Palatine, and had six corinthian columns on the front, preceded by a high staircase.
!Each year, in honor of goddess Cybele were held the Ludi Megalenses, which started
on April 4th and lasted six days. There were only theatrical performances and circus
games, with representations of comedies, banquets and gifts to the goddess. The Arch of
Light signaled the period of Ludi Megalenses and it is likely that the Pantheon was one of
the buildings in which were ofﬁciated ceremonies in honor of Cybele.
!In the late Summer, starting from September 2nd, was instead celebrated Jupiter
Optimus Maximus, who obviously was the most important deity of the Roman religion,
during the Roman Republic and the Empire. Along with Mars and Quirinus, Jupiter was
The diameter of the oculus
equals the width of the arch:
Fig. 14 - The diameter of the oculus and the with of the arch above entrance are the
same: nine meters (graphics processing from Ortophoto)
part of the most ancient Capitoline Triad, which later will include Jupiter, Juno and
!Jupiter was linked to the Italic god Diespiter, celestial deity who manifested himself
with the Sunlight during the day. Jupiter Optimus Maximus among other things was the
keeper of oaths, the protector of justice and of good governance. In his honor were
celebrated the Ludi Romani which initially foresaw horse races, to which were later also
added the Ludi circensi, with other races and sacriﬁces in honor of the god.
!The Ludi circenses were preceded by a solemn procession starting from the Temple
of Jupiter Optimus Maximus on the Capitol, which was the largest temple of the Capitoline
Hill. Dedicated to the Capitoline Triad, the temple was crowned with a bronze chariot led
by Jupiter, who was also portrayed in a grandiose chryselephantine statue placed inside,
similar to that of the temple of Zeus at Olympia, the work of Phidias. The temple, had six
colums on the front and a large staircase: was burned several times and restored by
Domitian after 80 AD. It was oriented towards south-east, and some monumental ruins
have been incorporated in the Palazzo dei Conservatori.
!Therefore, the Arch of Light of the Pantheon appears twice a year, on the occasion of
the great annual festivals dedicated to the two most important gods of the Roman
pantheon, a male - Jupiter - and a female - Cybele/Magna Mater. The Arch of Light
indicated the precise day on which they celebrated ritual feasts, processions and Ludi
dedicated to the two deities, and was one of the many 'indicators' of the passing of time in
this extraordinary building.
!The Arch of Light and the bright square on the ﬂoor also gave great prominence to
the Emperor and to the priests who entered the building during the ritual ceremonies. The
light of the Sun, in itself a symbol of divinity, gave them a magic and sacred aura (ﬁg. 15).
Fig. 15 - The Sun enters from the Oculus and hits the arch above the entrance, which interrupts the cornice
that runs at the base of the dome; on the right is the bronze original door (Photo by Francesco Lerteri)
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backstory.revealer.pantheon.cnn.html, minuto 4:18
Marina De Franceschini